Two short protein domains are responsible for the nuclear localization of the mouse spermine oxidase μ isoform

In mouse, at least two catalytically active splice variants (mSMO alpha and mSMO mu) of the flavin-containing spermine oxidase enzyme are present. We have demonstrated previously that the cytosolic mSMO alpha is the major isoform, while the mSMO mu enzyme is present in both nuclear and cytoplasmic compartments and has an extra protein domain corresponding to the additional exon Via. By amino acid sequence comparison and molecular modeling of mSMO proteins, we identified a second domain that is necessary for nuclear localization of the mSMO mu splice variant. A deletion mutant enzyme of this region was constructed to demonstrate its role in protein nuclear targeting by means of transient expression in the murine neuroblastoma cell line, N18TG2.